LOL, the tour that will never end. By the time it's finally over, every other band on the planet will have opened for GnR at some point.

When I saw them, there was no opening act.

They played for 3hrs and 45mins.

You probably saw them during their US arena leg near the end of 2017. I don't think they had an opening band that stretch. But most of the rest of the tour, they've had mostly pretty significant, well established openers that have included Alice In Chains, The Cult, ZZ Top, Wolfmother, Lenny Kravitz, The Struts, Babymetal, The Darkness, Rival Sons, Killing Joke, Live, Blackberry Smoke, Our Lady Peace, Deftones, etc. I think even The Who preceded them at some South American festivals.